I'm thinking of adding a 4 AWG wire to the cabin of my wife's SUV from the battery so as to be able to use a 750 watt inverter to its max*. In researching this I came across people installing amps attaching neutral to chassis. I know this is standard for wiring electronics in cars. My question is, is there a limit to the current carrying capacity of the chassis where we should use a wire to battery negative or is it a simple matter of always use chassis for negative even for potentially 750watt loads?

*I realize that the 750watt inverter only pulls the current needed for whatever load is plugged into it. I likely would only need a little over 300watts for a bottle warmer but even that exceeds the existing auxillary outlet wire so it just makes more sense, to me, that if I'm going to make a dedicated inverter wire run to the battery to use thick enough wire to support the inverter's max.

3 Answers
3

750 watts is slightly less than 63 amperes. So, the question is whether the car
chassis can supply 63 amperes.

Let's pick a 4 meters long steel object. If it weighs 500 kg (I assume a car
chassis weighs 500 kg, correct me if this is incorrect), its volume for density
8000 kg/m3 (different types of steel have different density, but 8000 kg/m3 is a rough approximation) is is 0.0625 m3 and its cross sectional area is then 0.015625 m2.

The electrical resistivity of carbon steel (NOT stainless steel) is 1.43e-7
ohm*m. This means the resistance of 0.015625 m2 cross section is 9.1520e-6
ohm/m, and for 4 meter long object it's 3.6608e-5 ohm. For a current of 63
amperes, it's 0.0023063 volt drop. In other words, 2.3 millivolts. Do you think
this is a problem? I don't think it is a problem.

The loss of power in watts is 0.145 watts, compared to 750 watts. Quite
minimal.

Now, how much copper do you need to achieve the same resistance? For 4 meters
long wire, you need 1.59e-8 ohm*m / A * 4 m = 3.6608e-5 ohm, solving the
equation gives A = 0.0017373 m2. In other words, 1737 mm2. Such a 4 meter long
copper wire would weigh 62.3 kg! My 5 meter long jumper cables are just 50 mm2.

I don't know if you consider 62.3 kg copper wire a problem, but I do. There's no way you can match the resistivity of the chassis with copper!

In summary: almost always you want to use the amount of metal in the car
chassis instead of installing separate wires. Even though steel does not have optimal resistivity, the sheer amount of metal in the car chassis overcomes the slightly less than optimal resistivity.

Edit: what is the current carrying capacity of the car's chassis? If you
consider 0.1 volt drop ok, it's 0.1 V / 3.6608e-5 ohm = 2732 amperes. This
would make the car chassis dispose of heat at 273 watts. I'm pretty sure you
could perhaps even double the voltage drop to 0.2 V, meaning 5463 amperes and
1.1 kilowatt heat disposal. I'm certain the car chassis can dispose of that
amount of heat.

@juhid You are assuming that all of the car body is transformed to a 4 meters long object with the cross sectional area as you calculated. That is much worse than reality. The return cable of the inverter will be much closer to the battery ground connection. OTOH I think 500 kg for a car body is way too heavy. If the SUV is on a chassis the picture changes even more. However, the starter motor (200A+) is also grounded to the chassis, as is the alternator (80A+). So your conclusion is still right. And the attempt to estimate the body resistance was very well done.
– Hans LinkelsJul 21 '18 at 22:33

I don't know automotive, but I do know electricity. Electrical noise may be a reason to use a dedicated cable as opposed to chassis ground. By putting the ground at the source (the battery), it's much more stable, especially if you also have the positive going straight to the battery. If you ground it further away, the varying load that might be demanded could cause other circuits within the car to see this variation on their inputs, and other things might cause noise on the inverter's input. Now ... it's an inverter ... It'll probably tolerate noise, but it may create a LOT, esp. when loaded.
– SteveJul 22 '18 at 0:27

That can be avoided other ways, such as using filter capacitors. The cable has weight, cost, difficulty of installation. Always trade-offs.
– SteveJul 22 '18 at 0:28

It sucks when noise gets coupled into the stereo or phone calls. Then I could see people complaining about some weird noise in the car and having no idea what it might be because all of the work is hidden. The inverter would cause a constant noise (probably 60 Hz and higher harmonics of that, maybe more since it probably produces a square wave and has strange demands on the source). Use the chassis, add caps if there is noise, use a cable as a last resort.
– SteveJul 22 '18 at 0:29

Is there ever a time where you'd use a separate wire to battery negative instead of chassis ground? As an aside, yes I would mount the fuse very close to the battery and use a battery disconnect inside the cabin so that the inverter can be removed easily without loose and exposed wires hanging about.
– Dean MacGregorJul 21 '18 at 18:19

Yes, did that to supply a large winch - both heavy duty cables directly off the batteries : 24v and special cables... Avoids having to change the standard vehicl chassis / battery connection, but for smaller stuff not always necessary.
– Solar MikeJul 21 '18 at 18:24

If you can find an easy spot to mount on the chassis, this is the best place to run the ground to for three reasons.

First, it's usually a lot more convenient than running your ground to the battery.

Second, it's a really good grounding place (as in solid and works well).

Thirdly, you run a lot less wire, which helps in cost and clutter.

According to this guide, you'll want to use 3AWG or larger wire. 4AWG wire is only rated to 60A for power transmission. 750 watts at 12vdc is 62.5A, which is more than what the 4AWG is rated for. You may actually want to run 2AWG wire for both if the length of run is longer than you're comfortable with. This is just an added measure of protection.

I think your guide is geared towards very long distances (or at least longer than is relevant for a car) based on it having a column for ohms per KM and because if you look up automotive AWG charts they have distance on one axis and current on the other. Based on the short distance that I would need automotive AWG charts suggest that 4AWG is fairly conservative.
– Dean MacGregorJul 22 '18 at 15:24

@DeanMacGregor - Obviously you can do as you choose. I was looking at industry standards for wire itself. This is pretty straight forward. You asked what wire you should you use for 750 watts, which 4 AWG is too small. Obviously for what you want to use it for (300 watts), 4 AWG should work just fine.
– Pᴀᴜʟsᴛᴇʀ2♦Jul 22 '18 at 15:54

Saying it's the "industry standard" doesn't really mean anything if the industry in question is power transmission instead of automotive. Here look at this or this or this.
– Dean MacGregorJul 22 '18 at 16:07

@DeanMacGregor - Well, even your first and last sites suggest 4 AWG is maxing out the wire for the task at hand. The second site seems to suggest there'd be no issue with the 4AWG. Even in my example, 4 AWG is right on the cusp of what the wire is rated to do. Will it work for your needs? I'd suggest absolutely (as I stated before). Will it work for full load? Most likely ... but I personally would still rather err on the side of caution. Much better than cooking something. Again, this is up to you.
– Pᴀᴜʟsᴛᴇʀ2♦Jul 22 '18 at 16:21

Hmm we seem to be differing on our assumption as to how long a wire needs to be to reach the cabin from the battery. My estimation is no more than 10 feet but probably less than 5. Perhaps I inadvertently gave the impression that I planned on having the inverter in the very back of the vehicle.
– Dean MacGregorJul 22 '18 at 18:54